[cfe-dev] Incongruency in __builtin_constant_p with pointer argument

[Abramo Bagnara]

Il 28/04/2011 18:47, Jonathan Sauer ha scritto:
> Hello,
> 
>>> From http://gcc.gnu.org/onlinedocs/gcc-4.4.5/gcc/Other-Builtins.html:
>>
>>> You can use the built-in function __builtin_constant_p to determine
>>> if a value is known to be constant at compile-time and hence that GCC
>>> can perform constant-folding on expressions involving that value. The
>>> argument of the function is the value to test. The function returns
>>> the integer 1 if the argument is known to be a compile-time constant
>>> and 0 if it is not known to be a compile-time constant. A return of 0
>>> does not indicate that the value is not a constant, but merely that
>>> GCC cannot prove it is a constant with the specified value of the -O
>>> option.
>>
>> I think that clang is right to consider pointer to static memory region
>> to be constant at compile-time and hence candidate to constant-folding.
>> (note that the documentation does not say "constant with a known value
>> at compile-time", but "value known to be constant at compile time").
> 
> But "candidate to constant-folding" requires the latter, I think. E.g. "a + 2" cannot be
> folded if the value of "a" is not known at compile-time, even if it happens to be constant.

$ cat t.c
int i;

int a[__builtin_constant_p(&i+1) ? 1 : -1];
$ clang -S t.c
$

clang disagrees...

What should be taken in account is that only the "numeric" value of the
pointer is not known at compile time, but its "symbolic" value is known.